/ Filters and sunglasses

I have some high quality sunglasses - Maui Jim and Adidas - which have great coatings that actually sharpen vision, don't just make everything duller. My understanding is that they work by cutting out some of the light wavelengths that don't focus quite on the retina. Subjectively they definitely enhance vision, and it's a disappointment to take them off - the world seems just a bit less sharp and colourful!

From what I can remember, some orangey ones do this (cut out blue I think). But don't think it would make much difference for a very good lens as they are usually corrected for colour aberrations with clever lens design.

In reply to Rob Exile Ward: The sunglasses aren't filtering out wavelengths that don't focus; the eye does the focusing for all incoming light.

It works either by polarizing the light, or by filtering for a specific colour.

Polarizers remove the 'disorganised' effect of light that has been scattered by reflections or atmospheric conditions, and produces clearer lines and less glare, which can also help you to perceive contrast and clarity.

Similarly, filtering for a colour can remove distractions and change the balance of an image. For instance, an orange filter will tend to make skies look a bit darker (because they are mainly blue and don't pass the filter), grass (which often glares) will be a touch darker because of its blue component, but earth and skin colours will stay fairly bright. B&W photographers used to use colour filters lots to change the balance of the picture, since the effect on colours was obviously not visible. These days you can do similar things in post processing, though some B&W specialists still use filters.

A UV filter is similar to the last type, and works by filtering out UV. This creates a purple, hazy light that dissipates over landscapes, particularly where there are particles or dust in the air. Scattered UV can have a 'veiling' effect that reduces contrast and apparent sharpness. Landscape photographers use these filters quite a bit to create crisp images with long sitelines.

I know what you mean - for cycling I have my 'happy-shades' which just make the world look a bit better.

You will come across a bit of a perception problem. As is often stated, you don't so must see with the eyes as with the eyes-brain combination. The brain is very good at 'correcting' for an amazing amount of defficiencies in the eyes (and glasses). You 'see' what your brain has learned after many years (and much evolution) to show you what it thinks you want or need to see. One aspect is that it's very good at ignoring colour casts and distortion. Try and experiment: put your glasses in front of your camera and see what the picture then looks like. They may work graet in front of your eyes-brain, but in front of a camera? Not so good. And the fact its those same eyes-brain that are looking at the picture afterward just doesn't work. The brain knows it's looking at a picture and doesn't know how to compensate.

There's no effective answer. A few filters may help a bit in very specific ways. UV was popular for film (which had extended UV sensitivity compared to eyes) but has little effect in digital cameras. Polarisers can cut down some glare, but have other undesirable effects (unnatural skys). There are expensive filters designed to cut out very specific wavelengths (like sodium lights), but that's not your problem. Sorry.

I don't think that's quite right, to be honest. Objectively check out the Adidas website, which will give you the relevant references, but basically YES they are filtering out ranges of specific wavelengths; subjectively check out a pair of their sunnies or comparable brand like Oakley or Maui Jim

> I don't think that's quite right, to be honest. Objectively check out the Adidas website, which will give you the relevant references, but basically YES they are filtering out ranges of specific wavelengths

What hamsforlegs wrote is a 100% correct and a very succint summary of the different effects of using filters.

You said "YES they are filtering out ranges of specific wavelengths" which is exactly the same as hamsforlegs' statement "...by filtering for a specific colour."

The wavelength of light is what determines its colour. When Adidas boast they are filtering different wavelengths, it is their marketing department trying to make "tinted lenses" sound more exciting.

To answer your original question: a polarising filter frequently has the effect of 'sharpening' an image due to increasing contrast. When paired with a yellow/orange filter (present in many sunglasses, especially Maui Jims) this can give the impression of 'sharpening' as you say. Especially when looking at foliage against a blue sky.

I understand wavelengths = colours, that's fine. And I understand that some wavelengths are refracted more than others through the same medium, which is how a prism separates the different colours.

Ergo, if an image is being focused through a lens onto a surface (retina, film, photo sensor) if some colours/wavelengths are focussed perfectly on to that surface then others - that are refracted more, or less - cannot be.

Obviously eyes have evolved to provide optimum focus across a range of light conditions and for a range of colours, which doesn't mean that they can't be improved upon in a specific set of circumstances, e.g. bright sunlight. But maybe I just need to experiment more with coloured filters.

> Ergo, if an image is being focused through a lens onto a surface (retina, film, photo sensor) if some colours/wavelengths are focussed perfectly on to that surface then others - that are refracted more, or less - cannot be.

Chromatic abberration is what Rob is indicating here I think. I think the human eye is actually very good at avoiding this, but I'm sure that wearing glasses that only let through a limited colour spectrum would remove any such issues if they exist. This isn't why glasses manufacturers tint their lenses though.

As in Interdit's post, chromatic abberration is normally addressed in photography through lens design and processing corrections rather than through filters.

I suspect that most problems with glare/contrast etc for the human eye are more to do with what happens 'out there' as light hits the viewed object and then makes its way to you via the atmosphere, and less to do with what happens 'in here' between your lens and your retina. But I'm not sure. Maybe the eyes do struggle with some kind of chromatic abberration effect?

In any case, the beneficial effects of sunglasses are down to the filtering effects I've described above, and you can achieve these effects using filters in photography.

The other thing that sunglasses do is stop mud, sticks, dusts and insects from wedging themselves in your eyes. Filters do a similar job for camera lenses.

> (In reply to interdit)
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> Good answer.
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> Chromatic abberration is what Rob is indicating here I think. I think the human eye is actually very good at avoiding this, ...

The eye is good at this because it only needs to be optically acute over a very small area of the retina (the fovea). For the vast majority of the field of view, the eye is actually pretty rubbish. Chromatic abberation is the least of its problems. Fortunately, the brain is very good at hiding this from you!